In a cellular system, mobile terminals are handed over between base stations. It is preferable that the change of the base-station occurs seamlessly for the user. In particular, seamless handover minimizes or at least reduces interruption time, and prevents data losses and sequencing issues.
The Long Term Evolution (LTE) System currently standardized in 3GPP—see TS 36.300 v8.0.0, 3GPP; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2, March 2007—provides for movement of network users between cells. More particularly, the specification includes Layer 2 (L2) provisions related to providing seamless handover. These L2 protocols include the Medium Access Control (MAC) protocol (with Hybrid Automatic Repeat Request (HARQ) provisions), the Radio Link Control (RLC) protocol (with ARQ provisions), and the Packet Data Convergence Protocol (PDCP).
In a known implementation, a (core) network gateway or other packet data entity delivers PDCP service data units to a base station for downlink delivery to a mobile station. The base station processes these service data units, such as by applying header compression and base-station specific ciphering, to obtain PDCP protocol data units that are transmitted in sequence over the air interface to the mobile station. The mobile station receives these PDCP protocol data units and processes them to recover the corresponding PDCP service data units. Notably, the protocol data units include sequence number information, which allows the mobile station to detect missed protocol data units and correspondingly reorder data as needed to ensure that the mobile station processes the service data units in their proper sequence.
Conversely, for uplink transmissions from the mobile station to the base station, the mobile station processes PDCP service data units originating at the mobile station, to obtain corresponding PDCP protocol data units, which are sequentially transmitted over the air interface to the base station. The base station processes these received PDCP protocol data units to obtain the corresponding PDCP service data units, and uses the sequence numbers to detect missing data and to conduct reordering as needed for proper sequential transfer of the uplink PDCP service data units to the core network.
Reception errors at the base station (and mobile station) can result in out-of-sequence reception. For example, the base station may receive two PDCP protocol data units in sequence, miss a third one, and successfully receive a fourth one. That fourth one cannot be processed for transfer to the core network until the missing third PDCP protocol data unit is successfully received at the base station. The base station thus buffers out-of-sequence data, holding it for reordering upon subsequent receipt of the missing data, which may be retransmitted according to known ARQ/HARQ processes in the MAC and RLC layers. The possibility of out-of-sequence data reception adds potentially significant complications to mobile station handover, where the mobile station is handed over from its current base station to a new base station.